1. Field of the Invention
The present invention pertains generally to the field of fasteners and is more particularly directed to a pull type blind fastener adapted for interference fit applications while providing grip thickness accommodation for the materials being joined and also providing for flush breaking pin positions.
2. State of the Prior Art
Fasteners that provide interference fit to the workpieces in which they are installed produce improved fatigue properties in the joint structure and provide a more rigid, tighter structure and produce leak-proof fastener installations. This is a rather easy task for bolts and the like which are simply forced into a structure hole which is a controlled amount smaller than the bolt diameter thus creating interference. These however, are not blind fasteners as the bolt is solid and guarantees retained interference as installed.
Blind fasteners on the other hand are multiple piece constructions that have as their primary components a hollow sleeve and a central pin portion moveable within the sleeve with or without additional parts and features that function to create a blind head on the inaccessible side of the workpiece upon installation. Blind fasteners adapted for interference fit conditions must in their installed condition act as a solid mass within the stretched installation hole in much the same manner as the solid bolt shank does to maintain the required interference fit.
The amount of interference required in the joint structure is important criteria and should be controlled from both high and low limits. The low limit is controlled such that the minimum level of desired interference is obtained in the joint for its structural benefits whereas the high limit must be controlled to the extent that the fasteners can be successfully installed and no damage to the structure occurs. Important criteria in the performance of blind fasteners in interference fit conditions are consistency of function and inspectability of installed units. Existing fastener designs suffer from significant drawbacks in these respects as illustrated in the following examples;
A first type is a variant of the popular NAS1675 threaded blind bolt series known commercially as Jo-Bolts or Visu-Loks. The variant consists of a tapered shank which when forced into a matingly prepared tapered hole in the structure produces an interference fit. The preparation of the specially reamed, tapered holes is a difficult and costly procedure. It is also difficult to inspect the accuracy of the prepared, tapered hole.
In application the tapered shank of the fastener is placed in the tapered hole wherein the manufactured head of the fastener sits slightly off the sheet line as a function of the interfering tapers. Upon installation the tapered shank must be forced into the tapered hole such that radially outward expanding interference is created and the fastener head seats flush with the workpiece. On these types of systems the force required to seat the fastener head and create the interference should be provided solely by the fasteners installation squeezing action as the blind head is formed since external pushing or bucking to seat the fastener can result in separation of the structure materials being joined. However, the fastener's installation squeezing action often lacks sufficient strength to fully seat the manufactured head and provide the proper level of interference. This results in improper fastener installations which must be removed and replaced, a practice that is particularly dangerous to the tapered holes Differences in structure materials or hardness conditions of same further aggravate the ability of the fastener to function in a consistent and predictable manner. A second type as exemplified by U.S. Pat. No. 3,820,297 to Hurd overcomes the problems associated with preparation of tapered holes. In this fastener the pull stem is drawn into the sleeve with a first interference fit. The resulting sleeve expansion radially outwardly creates a second interference fit to the structure hole in which it is installed.
Although these fasteners perform adequately their teachings and applications are narrowly limited to structure materials having strength levels approximate to or greater than that of the fastener sleeve member. Should these fasteners be installed in structures of significantly greater strength or hardness the sleeve will extrude before the structure hole will be stressed into interference. On the other hand should the same fasteners be installed in structures of significantly lower strength or hardness, the pin component will continue to move axially thru the sleeve, compressing and extruding the blind side of the work surface at the hole edge. Thus the strength of the structure materials controls significantly the proper operation of this device. In actual practice the fastener is fabricated to perform only in a specific structure condition by fine tuning installation loads, i.e. sleeve hardness, pin break load, lubricant coefficient of friction etc. Thus the fastener is suitable only for a specific material, material hardness condition and material thickness. It will not perform adequately if applied to other material types or hardness conditions.
An additional problem associated with fasteners of this type is that the pin break position on installed fasteners is controlled significantly by the structures blind side resistance to further travel of the pin. This means that variables in sheet thickness such as within the fasteners operative grip range result directly in variables in pin position protrusion on installed fasteners. These protrusions must be shaved off after installation to provide aerodynamic smoothness. The shaving operation is not only difficult and costly to perform on the high hardness alloy steel pins of these teachings but is also a dangerous practice that can lead to damaged structure skins. As noted the pin position is controlled significantly by the blind side sheet line. Another major contributing factor however to the installed fastener pin position is the frictional forces caused by drawing the pin into the interference fit. Fasteners of this type which lack positive mechanical stopping means, but rather rely heavily upon frictional forces to limit pin travel exhibit a significant degree of pin position inconsistencies. On blind fasteners it is particularly desirable to provide a means of inspection on the accessible side of the workpiece as an indicator of proper or improper blind head formations since otherwise the blind side configuration is not discernable to the installer. The use of a range of specified acceptable and un-acceptable pin positions as an inspection device for blind fasteners is a well known and accepted practice in the art. However, the combined affects of blind side sheet line variables and frictional forces on the pin positions of fasteners mentioned above severely compromise the integrity of their inspectability.
A final problem to be considered in the application of any blind fastener to be adapted for interference fit applications is that of compounding tolerance build-up conditions. By way of comparison a solid shank fastener such as a bolt must contend with only two tolerance factors when creating an interference fit, i.e. the tolerance of the bolt shank and the tolerance of the installation hole. Blind fasteners on the other hand, even in the most simple two piece constructions must contend with four compounding tolerance factors, i.e. pin shank, the sleeve I.D., the sleeve O.D. and finally the installation hole tolerance itself. Swings of tolerance build-up in one direction can cause over-interference and excessive fastener installation loading. Swings of tolerances in the opposite direction can prevent the proper interference from occurring and result in lessened fastener installation loads such that high, unacceptable pin positions can result. An attending problem is that blind fasteners are installation formed and the forming loads can therefore be compounded with loads created by swings in tolerances. Unlike solid shank fasteners such as bolts, blind fasteners must form their own head on the in-accessible side of the workpiece and typically provide for stem break-off. Thus blind fasteners have their own installation load characteristics with or without additional compounding loads that are caused by tolerance variables as applied to interference fit applications.
Efforts to produce predictable blind fasteners for interference fit applications have been frequently frustrated due to the sums of the compounding loads and fits involved. This has caused a need to tighten tolerances to a point where only highly precise and expensive manufacturing methods such as a reaming, honing and grinding can adequately achieve the specified limits. Even then swings of tolerances can occur within the tightly specified limits and cause fastener malfunctions. This is particularly true of fasteners that rely heavily upon friction to stop the pin at its proper break-off position.
A continuing need therefore exists for predictable flush breaking blind fasteners which provide a controlled amount of interference fit to the structures in which they are installed. A further need is that the fastener remain operative in different materials and material hardness conditions. Such an improved fastener should be simple and inexpensive to manufacture with a minimum number of parts. It is further desirable to make the fastener parts from raw materials and by manufacturing processes which minimize tolerance criticality problems.